build: upgrade Flink from 1.18.1 to 1.20.5

[romitshah02]

Upgrades Apache Flink from 1.18.1 to 1.20.5 across the full reactor (11 modules pinned via root pom.xml). Migrates every module off the deprecated flink-streaming-scala API (removed in Flink 2.0, FLINK-37248) onto the Java DataStream/StreamExecutionEnvironment/OutputTag/ProcessWindowFunction APIs, since job logic already used Java ProcessFunction/KeyedProcessFunction rather than Scala closures on Flink types.

Also bumps jobs-distribution/Dockerfile's base image and fs-plugin jar references from 1.18.1 to 1.20.5 (Flink dropped scala-suffixed image variants), and updates README.md's local-dev instructions (Flink download version, IntelliJ debug-mode temporary pom snippets) to match the new dependency shape.

Type of change

Please choose appropriate options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Test Configuration:

• Software versions: Flink 1.20.5, Java 11, Scala 2.12, Maven 3.8+
• Hardware versions: macOS (arm64)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-und
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my chan
• Any dependent changes have been merged and published in downstream modules

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